Carboniferous mineralisation, Wales

From Earthwise
Jump to navigation Jump to search

From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

The lead-zinc (Pb-Zn) mineralisation in Central Wales Mining District, between Aberystwyth and Llanidloes, is hosted mainly in Silurian and, less commonly, Ordovician rocks. However, lead isotope data indicates that the mineralising fluids were initially active in early Devonian times, possibly in response to the Acadian phase of the Caledonian Orogeny, and later in Carboniferous times, possibly prior to coalification events in south Wales. Similar associations occur in Silurian strata, in the Llanfair Talhaiarn district in Denbighshire, and in Ordovician strata in the Llanrwst district, on the west side of the Conwy valley.

In north-east Wales, there is extensive mineralisation along Variscan faults within Lower Carboniferous strata; on Halkyn Mountain and in the vicinity of Minera, the mineralisation has been intensively exploited. The deposits are mainly concentrated in the Lower Carboniferous limestones and the overlying Cefn y Fedw sandstone with lesser concentrations in the Basement Beds. The ores are mainly of sphalerite and galena, with little silver, associated with chalcopyrite, baryte and fluorspar. The ores are concentrated along joints, faults and, in the limestones, in cavities along bedding planes where the calcite has been dissolved. The ores tend to be richest where capped with a bed of shale, which restricted the upward movement of the solutions. The solutions are assumed to be magmatic, although their source remains problematic as there is no indication of contemporaneous igneous activity.

In south Wales, iron mineralisation in the Lower Carboniferous limestones has been extensively worked in the Vale of Glamorgan, near Llanharry. The ore consists mainly of colloidal and crystalline hematite, commonly altered to goethite, with quartz, calcite and dolomite. The ore bodies occur along the north-north-west-trending cross-faults. They fill large cavities and enlarged fractures in the limestones, generally beneath the Triassic unconformity, close to their contact with impermeable Namurian mudstones. The ores have been related to hydrothermal and to meteoric groundwater solutions. Probably both solution processes were involved — an earlier hydrothermal (sulphide) phase and a later replacement phase due to meteoric circulation. With the latter, the iron and silica are thought to have been leached from Triassic and Silesian strata, and deposited both as replacements and in cavities.


BARCLAY, W J. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Third edition. Memoir of the British Geological Survey. Sheet 232 (England and Wales).

DINELEY, D L. 1992. Devonian. 179–205 in Geology of England and Wales. DUFF, P MCL D, and SMITH, A J (editors). (London: The Geological Society of London.)

FRESHNEY, E C, and TAYLOR, E A. 1980. The Variscides of south-west Britain. 49–57 in United Kingdom introduction to general geology and guides to excursions. OWEN, T R (editor). Proceedings of the 26th International Congress, Paris, 1980.

GAYER, R, FOWLER, R, and DAVIES, G. 1997. Coal rank variations with depth related to major thrust detachments in the south Wales coalfield: implications for fluid flow and mineralization. 161–178 in European coal geology and technology. GAYER, R, and PES EK, J (editors). Geological Society Special Publication, No. 125.

GEORGE, T N. 1970. British regional geology: south Wales. Third edition. (London: HMSO.)

HAMPSON, G J, ELLIOTT, T, and DAVIES, S. 1997. The application of sequence stratigraphy to Upper Carboniferous fluvio-deltaic strata of the onshore UK and Ireland: implications for the southern North Sea. Journal of the Geological Society of London, Vol. 154, 719–733.

JONES, D G. 1074. The Namurian series in south Wales. 117–132 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff : University of Wales Press.)

KELLING, G. 1974. Upper Carboniferous sedimentation in South Wales. 185–224 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff: University of Wales Press.)

POWELL, C M. 1989. Structural controls on Palaeozoic basin evolution and inversion in south-west Wales. Journal of the Geological Society of London, Vol. 140, 439–446.

RAMSBOTTOM, W H C. 1973. Transgressions and regressions in the Dinantian: a new synthesis of British Dinantian stratigraphy. Proceedings of Yorkshire Geological Society, Vol. 39, 567–607.

THOMAS, L P. 1974. The Westphalian (Coal Measures) in south Wales. 133–160 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff: University of Wales Press.)

WARREN, P T, PRICE, D, NUTT, M J C, and SMITH, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir of the British Geological Survey, Sheet 107 and parts of sheets 94 and 106 (England and Wales).

WATERS, C N, BROWNE, M A E, DEAN, M T, and POWELL, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (onshore). British Geological Survey Research Report, RR/07/01.

WHITE, S. 1991. Palaeogeothermal profiling across the South Wales Coalfield. Proceedings of the Ussher Society, Vol. 7, 368–374.

WILSON, D, DAVIES, J R, FLETCHER, C N J, and SMITH, M. 1990. Geology of the south Wales Coalfield, Part VI, the country around Bridgend. Second edition. Memoir of the British Geological Survey, Sheets 261 and 262 (England and Wales).

WOODLAND, A W, and EVANS, W B. 1964. The geology of the south Wales Coalfield. Part IV. The country around Pontypridd and Maesteg. Memoir of the Geological Survey of Great Britain, Sheet 248 (England and Wales).

WRIGHT, V P. 1996. Use of palaeosols in sequence stratigraphy of peritidal carbonates. 63–74 in Sequence stratigraphy in British geology. HESSELBO, S P, and PARKINSON, D N (editors). Geological Society of London Special Publication, No. 103.